He helped make Ethel Merman a monster Momma in “Gypsy,” put words to Maria and Tony’s love in “West Side Story,” sized up the single life in “Company” — and, with “Sweeney Todd,” gave Johnny Depp something to sing about.

No wonder musical fans worship Stephen Sondheim. For more than 50 years, his words and music have made up an entire American songbook — though the feelings they express are universal.

The Manhattan native turned 80 on Monday, and the tributes are flowing faster than blood from Sweeney’s razor. Coming up: a star-studded birthday bash on April 26 at City Center; Roundabout’s “Sondheim on Sondheim” (through June 13 at Studio 54) and a rare revival of “Anyone Can Whistle” (City Center’s Encores!, April 8-11).

All the while, revivals of his “A Little Night Music” and “West Side Story” keep rolling merrily along.

Even Dame Edna relented. After declaring her “All About Me” musical a “Sondheim-free zone,” she delivered a delicious “Ladies Who Lunch” — the song that’s long kept Elaine Stritch in pantsuits.

What’s his secret? The Post asked some theatrical admirers.

“Look at those shows,” sighs David Hyde Pierce, who hosted the New York Philharmonic’s Sondheim salute. “Each one creates its own world so beautifully. That’s a very rare talent.”

And a self-deprecating one. Asked to perform in a small off-Broadway revue, the composer politely wrote back, “Thanks so much for the invitation, but I’d rather shoot myself in the foot.”

“He hates to perform,” confirms his longtime music director Paul Gemignani. What Sondheim likes to do, Gemignani says, is collaborate. “His whole purpose is to make something work, as opposed to, ‘This is my music. You do it my way.’ ”

Joanna Gleason, another Sondheim perennial, agrees. She’ll never forget how, when he was writing a solo for her character in “Into the Woods,” he called her at home to ask, “Any thoughts?”

“My jaw and the phone receiver both hit the ground,” she says. “Obviously, he had it all mapped out, but he was asking me what my character was feeling . . . Who does that? Who trusts actors that much?”

Cabaret goddess Barbara Cook, one of the stars of “Sondheim on Sondheim,” has been singing his songs nearly as long as he’s been writing them. To hear her tell it, he’s an amazing listener, a dazzling dinner guest — and a dream correspondent.

“He told me that his father said, ‘A gentleman always answers his mail,’ and so he says he answers every piece of mail that comes across his desk,” she says.

He can also be devastating. Just ask Patti LuPone, who was Momma Rose in Broadway’s latest “Gypsy” and Mrs. Lovett in a stripped-down “Sweeney Todd.”

This Broadway baby is nobody’s pushover, yet she sounds shaken recalling a rehearsal — for yet another Sondheim show, “Passion” — at which the composer “handed me my head.”

“He said, ‘Monotonous mush!’ He couldn’t understand a word I was saying. A lot of people have talked to me about my diction, but when it comes to Steve, there’s a wee bit of fear and a desire to get it right, because it’s his music!”

But even the music man gets blocked now and then. During the making of “A Little Night Music” in 1974, Len Cariou was Frederik, in search of “an 11th-hour song” — the big number at the end.

Sondheim was having trouble, Cariou says, so he and leading lady Glynis Johns improvised a scene for Sondheim to see.

“He said, ‘I think I know what to do now,’ and jumped in a cab,” says Cariou, who’ll appear at City Center’s tribute. “Three days later, he came back, sat down at the piano and said, ‘Sorry, Len, you don’t sing this anymore. She does.’ And he started to play, ‘Send in the Clowns.’ ”

Before “Ugly Betty,” Vanessa Williams played the Witch in the 2001 “Into the Woods” revival. She recalls waiting for a fax with director James Lapine for the direction for her final number. It came:

“Mr. Sondheim had the witch placing her bloody hand over the face of the stolen child and said he wanted the witch to start to eat the baby,” recalls Williams.

“James got on the phone and said, laughingly, ‘No, Steve, she’s not going to eat the baby!’ ” What may have been hard for Lapine to stomach can’t deter Williams, who clearly would do anything for Stephen Sondheim.

“If Mr. Sondheim does a new show that I’m lucky enough to be a part of,” she says, “I would gladly eat a baby onstage for him!”

As the ladies who lunch would say, we’ll drink to that.

HOT STAGE PICKS

By ELISABETH VINCENTELLI

WHEN it comes to Broadway, March and April are like Oscar season, Fashion Week and Black Friday sales rolled into one mad rush, as everybody scrambles to open before April 29 — the cutoff date for this year’s Tonys. Here’s a quick guide to this spring’s great white way: An import from London, the new two-hander “Red” (in previews, opens April 1) drills into the mind of painter Mark Rothko at the tail end of the 1950s — when he was commissioned to paint murals for the Four Seasons in New York. The show explores the creative process, but the real attraction is said to be Alfred Molina’s scenery-chewing performance.

Ken Ludwig’s breathless farce “Lend Me a Tenor” (in previews, opens April 4) returns to Broadway for the first time since 1989. Opera is a reliably great background for comic antics, but the producers aren’t taking any chances here: Stanley Tucci is directing Tony Shalhoub and Anthony LaPaglia.

On paper, “The Addams Family” (in previews, opens April 8) is a sure bet. Nathan Lane + Bebe Neuwirth + Charles Addams’ macabre characters: How could it go wrong? Don’t answer that — advance word on this new musical has been iffy, yet we’re still dying to see it. Again: Nathan, Bebe, etc.

“La Cage aux Folles” (previews start April 6, opens April 18) returns yet again, this time in an intimate British production that privileges the central gay relationship over the va-va-voom numbers. You can’t keep Jerry Herman’s score down though — it’d zing played on a ukulele.

At the opposite end of the musical spectrum is “American Idiot” (in previews, opens April 20), based on a Green Day album and directed by Michael “Spring Awakening” Mayer. What we’ve heard so far sounds like the kids from “Glee” singing the Ramones: punk fun for everyone!

Burt Bacharach and Hal David’s score alone is reason enough to attend the revival of the 1968 tuner “Promises, Promises” (previews start March 27, opens April 25). But there’s an added bonus: Kristin Chenoweth. She and Denzel Washington — who’s starring in August Wilson’s “Fences” (previews start April 14, opens April 26) — should generate enough star power to light up the tri-state area.

There aren’t big names in Lucy Prebble’s “Enron” (previews start April 8, opens April 27), which recounts the infamous financial debacle. Unless, that is, you count the likes of crooked CEO Jeffrey Skilling. If a play is as good as its villain, this one’s going to rock big time.